Process of extruding viscose wherein a formaldehyde containing acid bath solution is contacted with activated charcoal



United States Patent PROCESS OF EXTRUDING VISCOSE WHEREIN A FORMALDEHYDE CONTAINING ACID BATH SOLUTION IS CONTACTED WITH ACTIVATED CHARCOAL No Drawing. Application September 23, 1957 Serial No. 685,425

4 Claims. (Cl. 18--54) The present invention relates to a novel and useful process for the production of regenerated cellulose structures. More specifically it relates to an improvement in the conventional process for the preparation of regenerated cellulose shaped structures such as filaments, fibers, ribbons and the like, by extruding viscose into an aqueous acidic spinning bath.

It is known that the addition of certain low-molecular weight aldehydes, preferably formaldehyde, to the acidspinning baths used in the coagulation and regeneration of filaments by the viscose process leads to an increase in the stretch which may be imposed on the filaments during spinning and, hence, to an improvement in the strength of the ultimate product. A process of this type, employing formaldehyde, is described in U.S.P. 2,452,130, issued to G. M. A. Kayser on October 26, 1948.

Although the use of formaldehyde in the spinning bath is very desirable from the standpoint of improvements in the quality of the regenerated cellulose product obtained, serious difiiculties are encountered when the spinning is continued for long periods of time as must be done in the commercial practice of the process. After some hours of spinning a hard crystalline material begins to precipitate in the bath. These hard crystalline deposits are not to be confused with sludge formation due to metal sulfides, metal trithiocarbonates, free sulfur, etc., which occurs in most viscose spinning baths. These sludges cause some difliculty but may be dealt with by known methods. The hard crystalline material which deposits from the formaldehyde-acid spinning bath, forms on the wall of the bath lines and on other equipment Whenever the bath comes in contact with it. With continued spinning these hardened deposits build up and clog the bath lines and are very difficult, and sometimes almost impossible, to remove. Thus, processes of this type are commercially unattractive unless some means of alleviating this condition can be found.

It is an object of the present invention therefore to produce regenerated cellulose structures. Another object is the production of regenerated cellulose structures by an improved process. A still further object is the production of regenerated cellulose structures whereby crystalline deposits in the spinning bath are prevented. Other objects will become apparent hereinafter from the descriptions, the examples and the claims.

These objects are accomplished by the present invention which provides an improvement in the process of producing a shaped structure of regenerated cellulose by extruding viscose into a formaldehyde-containing acid bath, the improvement which comprises contacting the said formaldehyde-containing acid bath with activated charcoal. In the preferred embodiment of the invention the bath is continuously circulated through a bed of activated charcoal and such circulation is started within about one hour after starting extrusion.

While no limitation is intended as to any theory concerning the process of this invention, it has been found that the crystalline deposit consists primarily of trithiane. Apparently, the carbon disulfide and sodium hydroxide react so as to form sodium trithiocarbonate and some sodium sulfide. These compounds then react with the sulfuric acid in the spinning bath to form hydrogen sulfied and trithiocarbonic acid, and the trithiocarbonic acid decomposes to form hydrogen sulfide and carbon disulfide. Since it is known that hydrogen sulfide and formaldehyde react in acid solution to form hydroxy methyl mercaptan, it is believed that this then condenses to form trithiane. It is also believed that the trithiocarbonic acid reacts with formaldehyde to form bis-methylol trithiocarbonate which then decomposes to form the trithiane precursor hydroxy methyl mercaptan. The trithiane which is formed then precipitates out so as to form crystalline deposits. While these reactions appear to be the principal reactions in the formation of the crystalline deposit, other reactions may likewise take place so as to add to these deposits. In any event, the addition of the activated charcoal of the present invention in some way prevents such crystalline deposits from being formed as shown by the following examples. These examples are cited to illustrate the invention and are not intended to limit it in any manner.

Example 1 Viscose containing 5.0% recoverable cellulose and 6.5% total alkali, calculated as sodium hydroxide, is prepared in the conventional manner using 60% carbon disulfide based on the weight of air-dry cellulose pulp. This viscose is extruded into a spinning bath containing 10% sulfuric acid, 17.5% sodium sulfate, 9.5% Zinc sulfate and 0.7% formaldehyde, the temperature of the bath being 55 C. The yarn is led through the bath for a distance of 40 inches then stretched in a dilute acid, secondary bath at a temperature of 94 C. The tenacity of the yarn, which has a denier of 1100, after purification and drying is 4.85 grams per denier. On continuous spinning a hard-crystalline deposit (principally trithiane) is observed to form in the bath within 24 hours. In this test the bath system contained approximately 200 lbs. of spinning bath and the bath is circulated continuously in the usual manner.

The above testis repeated except that the spinning bath is circulated continuously through a bed of 14 x 20 mesh activated charcoal (marketed by Pittsburgh Coke and Chemical Co. as Type BPK). The bed of charcoal is 28 inches deep and is contained in a 4 inch diameter vertical column, wherein it is supported on a perforated lead disk covered with a plastic fabric composed of a vinylidene chloride polymer. A similar lead plate covered with this fabric is placed on top of the bed to confine the carbon and keep it from floating in the bath. The bath is fed upwardly through the column at a rate of about 1 lb. per minute. At the end of 48 hours continuous spinning no trithiane deposit is observed. The spinning stretch is and the yarn tenacity is 4.88 grams per denier.

When the formaldehyde is omitted from the spinning bath, continuous spinning is impossible at 117% stretch.

Example 2 In this example the composition of the spinning bath during extrusion is duplicated in order to test the effectiveness of activated charcoal under various conditions. This is done by preparing a solution containing sodium trithio carbonate and sodium sulfide in approximately the same concentrations as found in viscose, i.e., 0.61% sodium trithiocarbonate and 0.19% sodium sulfide. A sodium trithiocarbonate solution is prepared following the general procedure described by Weeldenburg, Rec. Tran. Chim. 47, 496 (1928). An excess of carbon disulfide is added to a 2.5 normal sodium hydroxide solution and the mixture stirred for 24 hours at room temperature. The excess carbon disulfide is removed by bubbling nitrogen through the solution and the solution, which contained 5.7% sodium trithiocarbonate, diluted with water to give a concentration of 0.61.% Sodium sulfide is added to the solution to give a 0.19% concentration of this compound. Fifty gram portions of this solution are added to four 910 gram samples of spinning bath containing 10.0% sulfuric acid, 17.5% sodium sulfate, 9.5% zinc sulfate and 0.75% formaldehyde, the temperature being 55 C. The samples are held at 55 C. for a total of 24 hours. At the end of 1 hour activated charcoal is added to three of the samples in amounts of 3, l0, and 50 grams respectively. -At the end of the 24 hour period the sample containing no charcoal produces a copious trithiane precipitate. --The sample containing 50 grams of activated charcoal produced no precipitate While those containing 3 and grams of activated charcoal produced a slight precipitate.

Example 3 Fifty grams of by-product sulfur solution prepared in Example 2 is added to 910 grams of spinning bath at 55 C. as described in Example 2. The sample is held at 55 C. for 1 hour and then poured through a column containing 100 grams of activated charcoal. The sample is then held at 55 C. for24 hours. No precipitate is observed.

Example 4 Fifty grams of activated charcoal isadded to one of two samples of spinning bath containing formaldehyde. The two samples, which are taken from the same bath and contained identical quantities of formaldehyde, are held at 55 C. for 24 hours and the concentration of formaldehyde in each sample determined at the end of this period. The activated charcoal-treated sample contains 0.59% formaldehyde at the end of the 24 hour period while the other sample contains 0. 61% formaldehyde. This indicates an extremely small adsorption of formaldehyde by the charcoal.

As shown by the foregoing examples, the formation of hard, crystalline deposits, consisting largely of trithiane, in formaldehyde-containing acid solutions which are exposed to hydrogen sulfide and/or trithiocarbonic acid may be prevented by bringing the solution into contact with activated charcoal after the solution has been exposed to the hydrogen sulfide and/ or the trithiocarbonic acid. This is conveniently accomplished by passing the solution through a single column of the charcoal, or alternating columns of charcoal (recharging the exhausted column) or in any other convenient manner.

It is thought that the charcoal removes the trithiane precursor, hydroxymethylmercaptan, which forms readily when formaldehyde is exposed to the action of hydrogen sulfide or trithiocarbonic acid, and thus prevents the formation of trithiane. Surprisingly the amount of formaldehyde adsorbed by the charcoal is negligible.

Preferably the acid solution is brought into contact with the charcoal within 1 hour after exposure to the hydrogen sulfide or trithiocarbonic acid. However, in some cases satisfactory results may be obtained when the charcoal treatment is delayed as much as 4 or more hours, depending on the temperature and other prevailing conditions. I

The amount of charcoal, relative to the volume of acid solution, which is required to obtain effective results will vary depending on the amount of formaldehyde present, the amount of hydrogen sulfide or trithiocarbonic acid present, the fineness of the charcoal and other conditions. It has been found, however, that even though minor amounts of activated charcoal will prevent the formation of minor amounts of crystalline deposits, amounts of about one pound of charcoal for each fifty pounds of spinning bath are preferred in order to substantially eliminate these deposits upon prolonged use.

Any of the viscose rayon spinning baths which are common to the industry may be used in conjunction with the present invention. Such baths usually contain 4 to 12% sulfuric acid, 5 to 25% sodium sulfate, and may also contain various other materials such as magnesium sulfate, zinc sulfate, iron sulfate, glucose, etc.

Many of the equivalent modifications will be apparent to those skilled in the art from a reading of the foregoing without departure from the inventive concept.

What is claimed is: v

1. In the process of producing a shaped structure of regenerated cellulose by extruding viscose into a formaldehyde-containing acid bath solution, the improvement which comprises contacting the acid formaldehyde-containing acid bath solution with activated charcoal to remove from the bath solution dissolved products which form hard crystalline deposits consisting largely of trithiane.

2. The process of claim 1 wherein the formaldehydecontaining acid bath is contacted with the activated charcoal by continuously circulating the bath through a bed of activated charcoal.

3. The process of claim 2 wherein circulation is started within about 4 hours after starting extrusion.

4. The process of claim 2 wherein circulation is started within about 1 hour after starting extrusion.

References Cited in the file of this patent UNITED STATES PATENTS Walker Dec. 5, 1944 

1. IN THE PROCESS OF PRODUCING A SHAPED STRUCTURE OF REGENERATED CELLULOSE BY EXTRUDING VISCOSE INTO A FORMALDEHYDE-CONTAINING ACID BATH SOLUTION, THE IMPROVEMENT WHICH COMPRISES CONTACTING THE ACID FORMALDEHYDE-CONTAINING ACID BATH SOLUTION WITH ACTIVATED CHARCOAL TO REMOVE FROM THE BATH SOLUTION DISSOLVED PRODUCTS WHICH FORM HARD CRYSTALLINE DEPOSITS CONSISTING LARGELY OF TRITHIANE. 